Tone control circuit



P 16, 1941- w. VAN B. ROBERTS 2,256,057

TONE CONTROL CIRCUIT Filed March 25, 1939 MED! UM FRERUENC/ES HIGH AZQZ/ENC/ES AF. (crass) INVENTOR 10w FREQUENCIES WAL rsizu B. ROBERTS Patented Sept. 16, 1941 TONE CONTROL CIRCUIT Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 23, 1939, Serial No. 263,603

3 Claims.

My present invention relates to tone control devices for audio frequency amplifiers, and more particularly to tone control devices capable of adjustment such that the low audio frequencies and the high audio frequencies may be individually enhanced, or reduced, without substantially affecting the amplification of the medium range of audio frequencies.

In its broadest aspect the object of the present invention is to provide, in an audio amplifier, means for affecting the gain at one extreme of a range of frequencies without thereby substantially affecting the gain at other portions of said range.

A particular object is to provide means to increase the gain at one frequency extreme without appreciably afiecting the gain at other frequencies.

' Another object is to provide means to decrease the gain at one frequency extreme of a frequency range without noticeably decreasing it at other frequencies.

Another object is to provide in a frequency discriminating amplifying system means for determining a frequency on one side of which there is substantially no frequency discrimination.

In its most particular aspect, the object of the present invention is to provide means for selectively increasing or decreasing the gain at audio frequencies below a predetermined middle range of frequencies, and independent means for selectively increasing or decreasing the gain of frequencies above said middle range.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows an audio circuit embodying the invention,

Figs. 2a to 2d inclusive show impedance variation characteristics of the tone control elements of the circuit shown in Fig. 1,

Fig. 3 illustrates various frequency-gain characteristics of the audio circuit.

Referring now to the accompanying drawing, the audio frequency transmission circuit in Fig. 1 embodies an amplifier I whose signal input grid 2 is adjustably connected by potentiometer slider 3 to a source of audio voltage 4. The source may be the output load resistor of a radio receiver detector; the electric pick-up of a record reproducer; the microphone output load of a public address system; the photo-cell pick-up of a sound moving picture system. In general, the source 4 may be any desired source of alternating voltage whose frequencies are located in the range of 0 to 15,000 cycles. The cathode of amplifier tube I is connected to ground through the usual selfbiasing network 5. The plate electrode 6 of tube I is coupled to the signal input grid 8 of the next amplifier 9 by the audio coupling condenser I.

A source of direct current Ill, and this current source may be the usual supply bleeder resistor of a power supply system, has its positive terminal connected to the plate 6 through a path including adjustable potentiometer slider I I, variable resistor I2, and variable resistor I3 of total resistance RL. The junction of resistors I2 and I3 is connected to a slider I4 by a coil I5. The junction of the resistors is also connected to the slider II by the condenser I6. The dotted line I'I denotes a mechanical coupling arrangement between sliders II and I4. The sliders are adjusted in unison, and in a manner to be explained at a later point.

The cathode of amplifier 9 is connected to ground, as in the case of amplifier I, by the selfbiasing network I8. The resistor I9 providees a direct current voltage path between grid 3 and the ground end of biasing resistor I8. The current source 20, which may be part of the source III, has its positive terminal connected to the plate 30 of amplifier 9 by a path comprising slider 2|, variable resistor 22, variable resistor 23 of total resistance RH. The junction of resistors 22 and 23 is connected to slider 25 by the condenser 24; and the same junction point is connected to tap 2| by coil 26. The dotted line 21 designates, again, a mechanical coupling device for adjusting sliders 25, 2i in unison.

The amplified audio voltage output of tube 8 may be transmitted to subsequent audio stages by the coupling condenser 28. The subsequent stage may comprise one or more amplifiers, or it may consist of any desired type of reproducer. The sliders I III are arranged so that resistance I2 is zero (that is, tap I I is maintained at the junction of resistor I2 and condenser IS) whenever the resistance value of resistor I3 is greater than zero. Similarly, tap I 4 maintains a short circuit across coil I5 so long as tap II is on any other part of I2 except its upper ends. In other words, coil I5 and condenser l6 are never both effective at the same time. This same relation exists for the adjustment of sliders 21-25.

With condenser l6 short-circuited, then for any given value R13 of that part of resistance R1. across coil (in other words the portion 13) the coupling impedance in the plate circuit of the first tube i is shown graphically as a function of frequency by the semi-circle in Fig. 2a. By a suitable choice of coil i5 relative to BL a very considerable reduction of this impedance is possible at low audio frequencies without substantial reduction of impedance at medium frequencies. For example, if R1. is equal to 200,600 ohms and the value of resistor section i3 is adjusted to 100,000 ohms, then a value of 100 henrys is sufficiently large to prevent any appreciable reduction of gain at audio frequencies above approximately 500 cycles, but cuts the gain at very low frequencies substantially in half. As tap it is moved upwards to increase R13 the high audio frequency gain is unaffected, but the very low frequency gain is still further reduced and the transition frequency, above which the gain is substantially unafiected, is increased.

On the other hand, for a given value of R12 (hence with slider i l at bottom of 53) Fig. 2b shows the variation of coupling impedance of tube 1! with frequency. In this case the condenser it and the maximum value of R12 are chosen relative to BL so that the very low audio frequency gain may be boosted a desired amount without appreciably affecting the gain in the medium range of frequencies. The smaller the value of R12 the less the low frequency boost and i the higher the transition frequency.

Figs. 2c and 2d show the impedance variations (mutually exclusive) resulting in the coupling network between tube 9 and the subsequent stage. In Fig. 20, with R22 equal to zero, adjusting tap upwards on resistor section 23 gives greater attenuation of high frequencies and A second knob can be employed for controlling taps 2i and 25 to secure the independent selection of the high audio frequency characteristics of Fig. 3. It will, therefore, be seen that there has been providedindependent control over the high and low audio notes thereby permitting the listener to adjust the tone to suit his wishes. The ordinates in Fig. 3 represent the total gain obtained by the two stage amplifier.

If a reduction in gain variation is not objectionable, it is possible to arrange both the high and low audio control networks in a single stage. It is, also, possible to omit one or more of the control units disclosed herein, but this is undesirable as there are times when each of the tone control units is needed. For example, the low frequency attenuation is often desirable to reduce boominess of male voices, while the low frequency accentuation is desired in many cases for bringing low frequency musical instruments up to proper relationship with other instruments when listening to music at low volume adjustment. As to the high frequency adjustments, the attenuation is. frequently desirable when listening to Weak distant signals through interference of nearly every type. The high frequency accentuation is desirable when listening to high fidelity signals free of interference when the receiving system discriminates against the high audio frequencies either by reason of a too sharply selective radio frequency system, or a loud speaker whose efficiency falls off at high frequencies.

While I haveindicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In an audio amplifier having input and output terminals, a passive coupling network connected across the amplifier between said input and output terminals and comprising a resistor element of fixed value, a reactance element adjustably connected to said resistor element to shunt an adjustable portion thereof, a second reactance element of opposite sign to the first reactance element arranged in series with said resistor element, a variable resistor element in shunt with said second reactance element, and uni-control means for varying the magnitudes of both resistor elements in successive order and in such a manner that at least one of said reactance elements is always short-circuited.

2. In an audio system of the type comprising a source of audio current of relatively high impedance, a passive coupling network connected across said source, and audio voltage output connections for taking off audio voltage from across the entire coupling network; the improvement which is characterized by said coupling network comprising a resistor element of fixed value, a reactance element adjustably connected to said resistor element to shunt an adjustable portion.

thereof, a second reactance element of opposite sign to the first reactance element in series with said resistor element, a variable resistor element in shunt with said second reactance element, and means for Varying the magnitudes of each of said resistor elements in successive orderand in such a manner that at least one of said reactance elements is always short-circuited.

3. In an audio signal voltage transmission cirouit having input and output terminals, apassive coupling network connected across said circuit between said input and output terminals and adapted for tone control, said network comprising a fixed coupling resistor, a reactance element having an impedance which increases with increasing frequency, a second reactance element. having an impedance which decreases with increasing frequency, one of said elements bein connected in series with said fixed resistor, the other element being connected in shunt to a variable portion of said fixed resistor, a variable resistor connected in shunt to said one element, said fixed and variable resistors being in series across said circuit, and uni-control means for successively adjusting the magnitudes of said portion and said variable resistor in such a manner that solely one of said reactance elements is effective at'any instant.

WALTER VAN B. ROBERTS. 

